Electric motors for powering downhole tools

ABSTRACT

An electric-motor assembly for installation in a borehole to power a downhole tools such as a pump has a first impermeable tube defining an axis and a stator fixed in the tube and having a series of coiled windings and laminations and electrically insulating potting in which the laminations and windings are imbedded. The windings have a connection to an electronically controlled power supply. An annular rotor in the tube radially surrounding the stator includes a permanent magnet coaxial with the windings. The tube seals the stator and rotor from the environment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of patent application Ser. No.10/528,664 filed 17 Mar. 2005 now U.S. Pat. No. 7,378,769 the USnational phase of PCT application PCT/GB2003/004009 filed 18 Sep. 2003with a claim to the priority of British patent application 0221630.7itself filed 18 Sep. 2002 and British patent application 0315848.2itself filed 7 Jul. 2003.

FIELD OF THE INVENTION

The present invention relates generally to downhole pumping systems and,more particularly to a new electric motor for use with a downhole toolssuch as a pumping system and that does not require a conventionalprotector.

BACKGROUND OF THE INVENTION

Electric submersible pumps (ESPs) are widely used throughout the worldfor bringing subterranean fluids to the earth's surface. For thelong-term successful operation of such submersible pumping systems, theelectric motor is supplied with uncontaminated motor oil. The motor oilnot only lubricates the motor, it also cools the motor to preventoverheating. In most submersible pumping systems in use today, thismotor oil is partially contained within a device commonly referred to asa motor protector. Conventional motor protectors typically include oneor more elastomeric bags. These elastomeric bags provide two importantfunctions: (1) equalizing the fluid pressure within the motor to that inthe adjacent wellbore and (2) preventing well fluids and gases fromcontaminating the motor oil. In regard to the first function, it shouldbe understood that the temperature of the motor oil varies as a resultof the intermittent operation of the submersible motor. As thetemperature of the motor oil rises, for instance, the oil tends toexpand and the pressure within the motor tends to increase. If the motorprotector did not include an expandable member, such as the elastomericmotor protector bag, the internal pressure of the motor would increasedramatically. However, the motor protector bag expands and contracts tocompensate for the varying liquid volume and to maintain a relativelyconstant pressure within the motor. In regard to the second function,the motor protector bag provides a degree of isolation between the motoroil and the well fluids and gases. This isolation helps keep the motoroil clean to increase the longevity of the motor. Most elastomeric motorprotector bags prevent many contaminants, such as crude oil, water,brine, and dirt, which may greatly reduce the life of the motor, fromentering the motor.

As discussed above, in many applications elastomeric motor protectorbags perform reasonably well. However, elastomeric bags suffer fromseveral limitations. First, the repeated expansion and contraction ofthe elastomeric bag can cause the bag to split or crack under certainconditions. Of course, once an elastomeric bag splits or cracks it nolonger protects the motor oil from contaminants that are then free toenter and ultimately damage the motor. Second, elastomeric bags tend tolose their elasticity due to various conditions that may be present in awellbore. Once an elastomeric bag loses its elasticity, it can no longerexpand and contract as needed to satisfy the requirements of the motoroil that it contains. Eventually the bag will rupture, leavingcontaminants free to attack the motor. Third, most elastomers cannotsurvive in environments where the temperature rises above about 400° F.Above that temperature, most elastomers become brittle, causing the bagto break during expansion or contraction. Finally, elastomeric compoundscurrently used for motor protector bags tend to be relatively permeableas compared to the contaminants within the wellbore fluid. Many wellscontain contaminants, such as hydrogen sulphide for instance, which willpermeate the motor protector bag and attack the motor. In fact, certaincontaminants, such as hydrogen sulphide, also tend to alter thechemistry of certain elastomers, causing the elastomers to harden. Oncethe elastomer has hardened, the bag eventually breaks. In an effort tocombat one or more these problems, the elastomeric material used tofabricate the motor protector bags have been studied and chosen toprovide certain advantages. For instance, certain elastomers may slowthe rate at which contaminants such as hydrogen sulphide enter themotor, but they cannot stop the permeation completely. Alternatively,certain elastomers may exhibit an ability to withstand temperatures ashigh as about 400° F., but these elastomers tend to have limitedelasticity incompatible with the requirements of the motor.

OBJECT OF THE INVENTION

The object of the invention is therefore to provide a new electric motorarrangement for powering downhole tools that avoids these problems withthe use of protector bags for protecting motors from the downholeenvironment.

SUMMARY OF THE INVENTION

According to the present invention there is provided an electric motorfor powering downhole tools comprising:

a first stator;

a second stator;

conductive windings;

an axially located rotatable shaft including a first magnetic elementand a second magnetic element; and

a sealed annular chamber defined by a first tube and a second tubeconcentrically inside the first tube, the first and second stators beinglocated in the annular chamber, the first magnetic element being alignedwith the first stator and the second magnetic element being aligned withthe second stator such that when the windings are energized the statorsact on the magnetic elements.

Preferably the conductive windings comprise a first set of coil windingsdisposed in the first stator and a second set of coil windings disposedin the second stator.

According to another aspect of the present invention, there is providedan electric motor suitable for installing in a borehole for poweringdownhole tools comprising

a stator including a first set of coil windings;

a rotatable shaft including a magnetic element; and

an annular cavity defined by a first hollow tube and a second tubeconcentrically inside the first tube, the second tube including aflowpath, the stators being located in the annular cavity, the rotatableshaft and the magnetic element being at least partially tubular.

BRIEF DESCRIPTION OF THE DRAWING

Several embodiments of the invention will now be described withreference to the following drawings in which:

FIG. 1 is a view of the general arrangement of an existing downholemotor used to power a pump;

FIG. 1 a is a view of the general arrangement of an embodiment of theinvention;

FIG. 2 is a longitudinal section of the typical prior art motor used inFIG. 1;

FIG. 3 is a longitudinal section of a motor according to a firstembodiment of the invention;

FIG. 4 is a longitudinal section of the motor of FIG. 3 with the rotorremoved;

FIG. 5 is a longitudinal section of a motor according to a secondembodiment of the invention;

FIG. 6 is a longitudinal section of a motor according to a thirdembodiment of the invention;

FIG. 7 is a transverse section through the motor of FIG. 6;

FIG. 8 is a longitudinal section of the motor of FIGS. 6 and 7 with therotor removed;

FIG. 9 a shows a side view of another embodiment of the modular motor;

FIG. 9 b shows a cross-sectional view of the rotor of this embodiment;

FIGS. 10 a and 10 b respectively show an exploded and assembled axiallysectional views of another embodiment of the modular motor;

FIG. 11 shows an axially sectional view of the several sections of anembodiment of the modular motor installed in a housing; and

FIG. 12 shows an axially sectional view of the several sections ofanother embodiment of the modular motor installed in a housing.

SPECIFIC DESCRIPTION

Where equivalent components appear in different embodiments, the samedesignating numeral will be used.

Referring initially to FIG. 1, a pumping system is illustrated andgenerally designated by a reference numeral 10. The pumping system 10 isshown located in a well bore 12 that has been created within asubterranean formation 14. Although not specifically illustrated, it iswell known that the well bore 12 contains fluids and gases from thesurrounding formation 14 and that the pumping system 10 is adapted to besubmerged in these fluids and gases within the well bore 12. The system10 is typically part of a production tubing string 16 and is responsiblefor pumping fluids and/or gases from the well bore 12 to the surface ofthe earth. The pumping system 10 includes a pump inlet 17 and a pump 18that is driven by an electric motor 20. The motor 20 contains motor oil(not shown) that lubricates and cools the motor 20. A motor protector 22is coupled to the motor 20. The motor protector 22 contains a portion ofthe motor oil, and it functions to keep the motor oil free fromcontaminants and to maintain a relatively constant pressure within themotor 20. The pump 18 then discharges through the tubing 11.

By contrast, referring to FIG. 1 a, according to an embodiment of thepresent invention a motor protector 22 may be included such as shownhere between the pump inlet 17 and the motor module 20, but can bedispensed with as will be described in greater detail below. Thecomponents are all generally mounted in a single housing 23. Two motormodules 20, 21 connected in series are attached via a top cap 13 to thetubing 11. Beneath the lowermost motor 21 is fitted the motor protector22. The pump 18 is situated beneath the motor protector 22, the pumpinlet being a bottom cap module 17. Several housing sections, eachassociated with the component it is housing, are secured together toform the complete housing 23.

FIG. 2 shows a longitudinal section through a conventional electricsubmersible pump (ESP) motor. These are induction motors that areessentially rotary transformers in which power transfers to thesecondary coil on the rotor, which results in rotation of a mechanicalload. The tolerance between the rotating and nonrotating componentsneeds to be quite close. The magnetic field is set up in the stator'smain inductance (the magnetizing inductance), which typically comprisesthree windings having a laminated soft-iron core 33. Most of the inputpower couples to the rotor secondary winding and thus the load. Therotor secondary winding also comprises three windings 27. The statorwindings are then driven by utility power via a pot head 29 in phasesseparated by 120 degrees. The result is a magnetic field that rotatesaround the motor axis at power frequency divided by the number of poles.Because there are windings on both rotating and nonrotating componentsand due to the close tolerance between the rotor and stator, they havealways had a common pressure compensated oil bath 22.

In FIGS. 3 and 4 we can see the first embodiment of the invention, usinga brushless DC motor 30 as opposed to the AC induction motors of theprior art. In these motors permanent magnets 31 are fitted to a rotor 32supported on rotor bearings 71, and as a consequence the clearancebetween the rotor 32 and the motor laminations 33 can be larger thanthat of an induction motor. In this embodiment a sleeve 34 (ofnon-magnetic stainless steel or a non-magnetic composite material tube)is arranged between the rotor 32 and the stator windings 73 and motorlaminations 33. This enables static O-ring seals 38, 39 to be arrangedbetween the sleeve 34 and the end fitting 35 isolating the laminationand windings section of the motor from the rotating sections of themotor and pump. As can best be seen from FIG. 4 a sealed annular chamber36 is created between the outer housing 37 and the inner sleeve 34 inwhich the motor laminations 33 and connections are located. The sealedchamber 36 is defined at each end by the end fittings 35 and thecorresponding O-rings seals 38, 39 provided on the internal wall of thehousing 37 and the external wall of the sleeve 34. The seals 38 and 39do not need to seal against rotational movement but merely to sealagainst a degree of lateral movement required to compensate for pressureand temperature variations. These seals 38 and 39 are therefore muchmore reliable and less costly than rotating seals. This sealed chamber36 is completely isolated from the oil well environment.

This provides the following significant advantages

1. No rotating seals are required to isolate the water and gas-sensitivelaminations, electrically insulated windings and electrical contacts.

2. Hydrogen sulphide cannot enter the motor oil past the static seal, soscavengers need not be added to the motor oil. The lack of scavengers isadvantageous for various reasons. For instance, motor oil additives,such as scavengers, tend to increase the cost of the motor oil. Also,such additives typically reduce the effectiveness of the motor oil inperforming its primary functions of cooling and lubricating the motor.Finally, it has been found that many such scavengers reduce thedielectric constant of the motor oil. In the event that insulation thatprotects windings and other conductors within the motor fails, a motoroil having a high dielectric constant is advantageous because it willreduce the likelihood of arcing between exposed conductors that maydamage the motor.

3. A simple oil expansion and contraction system can be used that iswell proven and understood, and again only has non-rotating seals.

Referring now to FIG. 5 a modification of the embodiment in FIGS. 3 and4 is shown in which the sealed chamber 36 includes a hydrostatic andtemperature/pressure compensation means 40 that allows for the effectsof the large pressure and temperature changes that the sealed chamberwill be subject to ensuring that no pressure difference builds up thatcould damage the seals 38, 39, housing 37 or sleeve 34. The compensationmeans includes a laterally movable plug 41 in sealing engagement withthe inside walls of the chamber 36 that forms a compensation chamber 42having a vent hole 43 through the housing 37 to outside the motor.

Referring now to FIGS. 6 to 8 a further embodiment is shown in which therotor 32 is arranged outside the laminations 33 allowing flow of fluidsthrough the empty bore of the motor. The same reference numerals in allembodiments are used for corresponding elements even where thearrangement of them is different. The rotor 32 and the permanent magnet31 are arranged on the outside of the windings 73 and laminations 33,and the windings 73 and laminations 33 are similarly enclosed in astainless-steel sleeve 34 arranged between the permanent magnet 31 onthe one hand and the windings and laminations 33 on the other. Thelaminations 33 are arranged in annular formation around a tubularinternal housing 50 through which pumped fluids flow. A sealed chamber42 is formed between the internal housing 50 and the sleeveencapsulating the laminations 33 and protecting them from a well fluids.The sealed chamber 42 contains a pressure compensation means 52 thatserves to adjust to any pressure changes outside the motor through venthole 43. This can be seen best from FIG. 8 that shows this embodimentwith the rotor removed.

FIGS. 9 a and 9 b shows details of another embodiment of the modularelectrical motor. A sleeve 34 (of nonmagnetic stainless steel or anonmagnetic composite material tube) is inserted into the bore of thestator's windings and laminations. The sleeve 34 is mounted on an endfitting 35 that has passages 76 for the windings' electric power supplycables 77. A magnetic bearing element 47 including magnets 80 is alsofitted over the sleeve 34, against the end fitting 35. The rotor 32 isintroduced inside the sleeve 34, so that sleeve 34 separates thestator's laminations 33 and windings 73 from the rotor. The rotor 32comprises a tube having a flowpath, the tube including a permanentmagnets 83 potted in resin 84 upon its surface, protected by a thinnon-magnetic sleeve 86. When such motor modules are assembled in seriesas will be described below, each magnetic bearing element 80 acts on aferrous portion 48 of adjoining rotors 32, so that each rotor 32 has twosuch magnetic bearing elements at either end to support it.

Referring to FIGS. 10 a and 10 b, the end fitting 35 features magnets 80that form a radial bearing acting to center the rotor 32 while allowingit to rotate. The rotor 32 also includes a magnetically susceptibleflange 88. Magnetic thrust bearing elements 90 are fitted to the endfitting 35, and these act on the flange 88 to constrain the rotoragainst axial displacement. An intermediate member 89 then fits over thenon-magnetic sleeve 34 to secure the module's assembly. A further radialbearing may be included at the forward end of the rotor if desired.

The rotor here also includes magnetic elements 29 that are configured tooppose the magnets 80 of the magnetic bearing element 48. Similarly,such opposing magnets may be included at the other end of the rotor, andin the rotor flange to act with the magnetic thrust bearing 90.

Referring to FIG. 11, several motor modules 60 can be arranged in aseries (and the windings can be electrically connected in series); twosuch motor modules are shown here. A modular motor enables a moreefficient and productive construction. After each module has beenassembled, the adjacent module's electric power supply cables 77 areconnected. The module are then pushed together so that the nose of onerotor 92 enters the mouth 94 of the adjacent module's rotor. Ideally,the rotors include interlocking splines so that the shaft as a whole cantransmit torque satisfactorily. A seal 93 on each rotor isolates theflowpath 81 from the module components around the rotor. An outer tube96 has already been secured to the stator's windings 73 and laminations33 and to part of the end fitting 35. As the modules are pressedtogether, the part of the end fitting not already secured to the outertube 96 enters the outer tube 96 of the adjacent module, and is secured,as will be described in more detail below. An end cap 102 having aradial bearing 104 and a bore to continue the flowpath 81 through therotor is also secured to the last motor module. The module componentsare then fixed in position by swaging parts of the outside of the outertube 96, particularly in the region of each end fitting and the end cap.This swaging not only secures the outer tube to the end fitting, but mayalso be utilized to secure the end fitting to the bearing, the rotorends to one another, and a seal block 104 to the sleeve 34. This sealblock, in addition to the rotor seal 93, helps isolate the common volumecontaining all the motor modules' windings 73 and laminations 33.

This embodiment shows the rotor 32 supported by conventional rollerbearings 104 that constrain the rotor both radial and axially.

Referring to FIG. 12, a series of motor modules 60 may be securedtogether before being collectively inserted into a single length ofouter tube 106. The tube 106 is then swaged in order to secure itself tothe motor modules 60. In this embodiment, the bearings shown aremagnetic radial and thrust bearings 90, 80. The thrust bearings mayincorporate seals 108.

The motor modules 60 could be arranged using a single rotor extendingthrough the modular system. If necessary, parts of the rotor,particularly if being used with conventional bearings, could be securedby introducing a swaging die inside the flowpath of the rotor andincreasing the rotor's diameter. Swaging techniques, both of the outertube and the flowpath of the rotor, may include not only increasing theentire the circumference of part of the tube, but also radialdeformation of relatively small regions, such as pressing small dimplesinto the outer tube to secure it to the motor modules 60 inside.

As in the previous embodiments, this configuration provides thepreviously discussed significant advantages with regards to theisolating the gas-sensitive laminations, electrically insulated windingsand electrical contacts without recourse to rotating seals, the need forscavengers added to the motor oil is obviated since contamination byhydrogen sulphide is eliminated, and simple oil expansion andcontraction systems are well proven and understood, and again only hasnon-rotating seals.

Rather than fabricate each set of coil windings from one cable, and thenconnected the completed coil windings when the motor modules are placedin series, a single coil winding may be threaded around each set oflaminations in successive stators. To this end, a single coil couldextend along each stator until the last stator is reached before beingfolded back and extending along each stator in the opposite direction.Another manner of supplying the stator is to dispose axially orientedcables in the laminations, before connecting the ends of pairs cables soas to form a conductive coiled path.

While the invention may be susceptible to various modification andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been be described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. An electric-motor assembly for installation in a borehole to power adownhole tool, the assembly comprising a first impermeable tube definingan axis; a stator fixed in the tube and having a series of coiledwindings and laminations and electrically insulating potting in whichthe laminations and windings are imbedded, the windings having aconnection to an electronically controlled power supply, the statorforming a flowpath that extends through the stator; and an annular rotorin the tube radially surrounding the stator and including a permanentmagnet coaxial with the windings, the tube sealing the stator and rotorfrom the environment.
 2. The electric-motor assembly defined in claim 1wherein the potting material is impervious to wellbore fluids.
 3. Theelectric-motor assembly defined in claim 1 wherein the potting materialis introduced under a vacuum.
 4. The electric-motor assembly comprisinga second such assembly as defined in claim 1 with the two assembliesconnected in series.
 5. The electric-motor assembly defined in claim 4wherein the electric-motor assemblies are secured together and bothimbedded in the potting material.
 6. An electric-motor assembly forinstallation in a borehole to power a downhole tool the assemblycomprising: a first impermeable tube defining an axis; a stator fixed inthe tube and having a series of coiled windings and laminations andelectrically insulating potting in which the laminations and windingsare imbedded, the windings having a connection to an electronicallycontrolled power supply; an annular rotor in the tube radiallysurrounding the stator and including a permanent magnet coaxial with thewindings, the tube sealing the stator and rotor from the environment; asecond tube coaxial with the first tube; and annular end members thatsubstantially seal an annular space between the first and second tubes.7. The electric-motor assembly defined in claim 6 wherein the annularend members are attached to the first and second tubes by swaging.
 8. Anelectric-motor assembly for installation in a borehole to power adownhole tool, the assembly comprising: a first impermeable tubedefining an axis; a stator fixed in the tube and having a series ofcoiled windings and laminations and electrically insulating potting inwhich the laminations and windings are imbedded, the windings having aconnection to an electronically controlled power supply; an annularrotor in the tube radially surrounding the stator and including apermanent magnet coaxial with the windings, the tube sealing the statorand rotor from the environment; a metal tube set the potting; an O-ringin the metal tube; and wiring exiting from the potted material throughthe O-ring and metal tube.